INFICON Maxtek PM-700 Plating Monitor - Manual (Page 75)

PM-700 SERIES PLATING MONITOR
67
6. APPENDIX
6.1 THEORY OF THE QUARTZ CRYSTAL MICROBALANCE
6.1.1 Crystal Frequency
The monitor uses a quartz crystal as the basic transducing element. The quartz
crystal itself is a flat circular plate approximately one inch (2.5 cm) in diameter
and 0.011 to .0132 inches (.028-.033 CM) thick for 6 and 5 MHz. The crystal is
excited into mechanical motion by an external oscillator. The unloaded crystal
vibrates in the thickness shear mode at a frequency of approximately 5 or 6
Megahertz depending on crystal selected.
The frequency at which the quartz crystal oscillates is lowered by the addition of
material to its surface.
6.1.2 Crystal Health Calculation
Crystal health decreases from a value of 100% for an uncoated crystal blank to 0%
at a total coated areal mass of 25 milligrams per square centimeter. The above
value corresponds to a crystal frequency shift of approximately 1.5MHz, or an
aluminum thickness of 92.5 Microns.
Since very few materials can be deposited to this thickness without producing a
crystal failure, a crystal health of zero will not normally be achieved; indeed, for
some materials the crystal health may never get below 90%.
In order to establish the point at which the crystal should be changed, or stripped,
several trial runs should be made to determine the point at which the crystal fails
(O FAIL message flashes) and subsequent crystals should then be replaced or
stripped well in advance of this point.
Because the crystal health is determined from the calculated material mass, the
Acoustic Impedance parameter will affect the displayed crystal health.
6.1.3 Thickness Calculation
Early investigators noted that if one assumed that the addition of material to the
crystal surface produced the same effect as the addition of an equal mass of
quartz, the following equation could be used to relate the film thickness to the
change in crystal frequency.
where: N
q
= 1.668 X 10
5
cm/sec.
Frequency constant for an AT cut quartz
( )
TK
N
f
f f
f
q q
f
q
=
⋅
⋅
−
ρ
ρ
2
(1)